1. Field of the Invention
The present invention relates to a lithium secondary battery and, more particularly, to a lithium secondary battery in which the construction of a negative electrode is improved.
2. Description of the Related Art
Recently, nonaqueous electrolyte batteries using lithium as a negative electrode active material have attracted attention as high-energy-density batteries. Nonaqueous electrolyte primary batteries using, e.g., manganese dioxide (MnO.sub.2), carbon fluoride [(CF.sub.2).sub.n ], and thionyl chloride (SO.sub.3 Cl.sub.2) as a positive electrode active material have already been used extensively as power supplies of pocket calculators and watches and as backup batteries of memories.
In addition, with decreasing size and weight of various electronic devices such as VTRs and communication devices in recent years, demands have increasingly arisen for high-energy-density secondary batteries for use as power supplies of these devices. Therefore, the study of lithium secondary batteries using lithium as a negative electrode active material is being vigorously done.
More specifically, the study is being made on a lithium secondary battery using a negative electrode such as metal lithium; a nonaqueous electrolyte prepared by dissolving a lithium salt, such as LiClO.sub.4, LiBF.sub.4, or LiAsF.sub.6, in a nonaqueous solvent, such as propylenecarbonate (PC), 1,2-dimethoxyethane (DME), .gamma.-butylolactone (.gamma.-BL), or tetrahydrofuran (THF), or an electrolyte, such as a lithium ion conductive solid electrolytic salt; and a positive electrode active material, such as a compound which causes a topochemical reaction with lithium, e.g., TiS.sub.2, MoS.sub.2, V.sub.2 0.sub.5, V.sub.6 0.sub.13, and MnO.sub.2.
However, no such lithium secondary batteries as discussed above have been put into practical use. The major reasons for this are a low charge/discharge efficiency and a small number of times (a short cycle life) by which charge/discharge is possible. This is considered to be caused primarily by deterioration of lithium brought about by the reaction between the metal lithium of the negative electrode and the nonaqueous electrolyte. That is, lithium which has dissolved as lithium ions in a nonaqueous electrolyte during discharge reacts with the nonaqueous solvent of the electrolyte in precipitating on the surface of a lithium negative electrode during charge. This makes a portion of the surface of the negative electrode inert. Therefore, if the charge/discharge is repeatedly performed, lithium precipitates in a dendrite form or in the form of small spheres on the negative electrode surface and is eventually separated from the collector.
For this reason, there has been proposed a lithium secondary battery including a negative electrode containing a carbonaceous material, such as coke, a resin calcined body, carbon fiber, or thermally decomposed vapor-phase carbon, which absorbs and desorbs lithium ions. Since the negative electrode contains the carbonaceous material, this secondary battery can discourage the reaction between lithium ions and the solvent of an electrolyte occurring when the lithium ions precipitate on the surface of the negative electrode. This makes it possible to prevent a portion of the surface of the negative electrode from being rendered inert. Consequently, degradation in the negative electrode characteristics caused by dendrite precipitation can be reduced. However, since the negative electrode containing the carbonaceous material has only a small lithium ion absorption/desorption amount, its negative electrode specific capacity (unit; mAh/g or mAh/cm.sup.3) is also small. Additionally, if the lithium ion absorption amount is increased, i.e., if the charge capacity is increased, the structure of the carbonaceous material deteriorates, or the solvent in a nonaqueous electrolyte decomposes. Furthermore, if the charge current is increased, the lithium ion absorption amount decreases, and metal lithium precipitates on the surface of the negative electrode. Therefore, the cycle life of a lithium secondary battery incorporating the above negative electrode is shortened.
On the other hand, it has been attempted recently to use a heat-treated material of a .pi.-electron conjugated polymer, such as polyacene, as the material of the negative electrode of a lithium secondary battery. However, the capacity of a lithium secondary battery incorporating a negative electrode containing this material is low. In addition, the cycle life is shortened if the capacity is increased.